Replace static numpy allocator by function containing static.

That enables the numpy code to be its own library, in case
some users want to (e.g. CLIF library).

modules/python/src2/cv2_numpy.cpp

@@ -6,8 +6,6 @@
 #include "cv2_numpy.hpp"
 #include "cv2_util.hpp"
 
-NumpyAllocator g_numpyAllocator;
-
 using namespace cv;
 
 UMatData* NumpyAllocator::allocate(PyObject* o, int dims, const int* sizes, int type, size_t* step) const